Add an author_style_disabled flag to stylist.update, and associated structs.

MozReview-Commit-ID: FiXyEN4xVnU

components/layout_thread/lib.rs

@@ -1085,6 +1085,7 @@ impl LayoutThread {
             ua_or_user: &ua_or_user_guard,
         };
         let mut extra_data = ExtraStyleData {
+            author_style_disabled: None,
             marker: PhantomData,
         };
         let needs_dirtying = Arc::get_mut(&mut rw_data.stylist).unwrap().update(